Prior art X-ray tubes wherein the location of the focal spot on the anode is controlled, includes the tube described in U.S. Pat. No. 4,689,809 assigned to the Assignee of the present invention. Therein the focal spot is controlled and located successively at two different locations. That X-ray tube with the dual focal spot location is particularly useful in computerized tomographic scanners since it effectively doubles the number of X-ray beams used during the scan thereby increasing the resolution.
One of the problems with X-ray tubes, especially those used in computerized tomographic scanners, is that external forces act on the electron stream coming from the cathode of the X-ray tube and terminating at the focal spot on the anode of the X-ray. Thus, for example, the stream of electrons themselves generate a magnetic field. The magnetic field of the stream of electrons is influenced by the earth's magnetic field as the X-ray tube on the rotor of the CT tomographic scanner rotates about the patient. The earth's magnetic field causes a perceptible deflection of the position of the focal spot on the anode during the rotation of the X-ray tube.
All uncontrolled changes in position of the focal spot on the anode are highly undesirable in computerized tomography wherein the position of the radiation source is part of the basic algorithm for processing the data to obtain the image. Many times CT systems are proximate to magnetic resonance imaging (MRS) equipment. Stray magnetic fields from the MRI equipment may also cause variations in the location of the focal spot.
In addition to the migration of the focal spot due to external magnetic fields, the migration of the focal spot in conventional X-ray tubes occurs for other reasons such as thermal expansion of components of the tube during operation and/or due to vibrations of the tube components induced by the mechanical rotation of the anode of the X-ray tube. Also, during the life of the X-ray tube, which may extend to over 100,000 scans (exposures) the cathode structure may move slightly from its original position, or the elements of the cathode structure may move slightly with respect to each other. The location of the focal spot is also adversely influenced by thin films which often form on the surfaces in the tube causing drift of the focal spot position.
The above mentioned U.S. patent does not provide a solution to the problems caused by the movement of the focal spot. Rather it provides dual focal spots each of which may migrate or be deflected for the reasons noted immediately hereinabove.
A U.S. Pat. No. 4,819,260 provides a system for controlling the location of the focal spot to maintain that location stationary. However, in that patent a detector at the edge of the X-ray beam is used to determine movement of the focal spot. In addition the control of the focal spot is accomplished magnetically. Having a detector at the edge of the X-ray beam is limiting. One of the problems inherent to the magnetic control of the focal spot is that it becomes necessary to use non-magnetic material in manufacturing the cathode head and the anode.
Another problem with the magnetic field control of the location of the focal spot is that a relatively large coil has to be attached externally to the tube. This increases the size and the cost of an already expensive and relatively large item in the CT scanner.